The present invention is directed to bicycles and, more particularly, to a shift positioning device for positioning the actuating member of a bicycle transmission.
Bicycle transmissions include internal transmissions and external transmissions. Internal transmissions ordinarily include a planetary gear mechanism fitted in the rear wheel hub. External transmissions ordinarily have a multiple sprocket cassette mounted on the rear wheel hub or crank, and a derailleur guides a chain among the multiple sprockets. In either case, a shift control cable is connected to the transmission and to a shift control device mounted to the bicycle frame. In the past, the rider would operate the shift control device to select a desired transmission gear.
Recently, automatic bicycle transmissions have been developed which automatically shift the transmission to higher gear ratios when the bicycle is traveling at high speeds and which automatically shift the transmission to lower gear ratios when the bicycle is traveling at low speeds. Such automatic transmissions usually are equipped with an electric motor for operating the shift control cable, a speed sensor for sensing bicycle speed, a shift control device containing a microcomputer for controlling the electric motor in response to bicycle speed, and a case mounted to the bicycle frame for containing these components.
A shift positioning device for a known shift control device has a base member disposed inside the case, a winding body rotatably coupled to the base member for winding the shift control cable, a positioning member retained to the winding body and allowed to move in the axial direction, a releasing member disposed adjacent to the positioning member and rotatably mounted on the base member, an electric motor, and a gear reduction mechanism for stepping down the revolutions of the electric motor and transmitting them to the releasing member. The positioning member is biased axially against the releasing member by a biasing member. The releasing member is ultimately connected to the winding body while allowed to have a specific play in the direction of rotation.
The positioning member and the base member have mating surfaces facing each other in the axial direction and form a positioning mechanism for positioning the positioning member according to the speed steps. The positioning member and the releasing member also have mating surfaces facing each other in the axial direction to form a unidirectional interlocking mechanism so that the positioning member, the releasing member and the winding body rotate as a unit when the releasing member rotates in the winding direction of the shift control cable. The unidirectional interlocking mechanism also releases the positioning member from its current position when the releasing member rotates in the unwinding direction of the shift control cable so that the winding body moves in the unwinding direction of the shift control cable. The unidirectional interlocking mechanism and the positioning mechanism both comprise a plurality of right-triangular cam surfaces slanted in opposite directions, and they are formed such that the cam surfaces of the unidirectional interlocking mechanism are higher than the cam surfaces of the positioning mechanism.
When the releasing member is rotated in the cable winding direction by the electric motor in a mechanism thus configured, the releasing member rotates the positioning member and causes the cam surfaces of the positioning mechanism of the positioning member to travel over the cam surfaces of the positioning mechanism of the base member. Thereafter, the cam surfaces of the positioning mechanism of the positioning member again engage the cam surfaces of the positioning mechanism of the base member, but this time the positioning member and therefore the winding body have rotated the equivalent of one speed step in the cable pulling direction. The shift control cable is likewise pulled the equivalent of one speed step.
When the releasing member is rotated in the cable unwinding direction, the positioning member is moved in the axial direction by the cam surfaces of the unidirectional interlocking mechanism, and the positioning member is released from the base member because the cam surfaces of the unidirectional interlocking mechanism are higher than the cam surfaces of the positioning mechanism. Thereafter, the cam surfaces of the positioning mechanism of the positioning member again engage the cam surfaces of the positioning mechanism of the base member, but this time the positioning member and therefore the winding body have rotated the equivalent of one speed step in the cable unwinding direction. The shift control cable is likewise released the equivalent of one speed step.
In such devices, the position of the winding member is controlled by axial movement of a positioning member relative to a releasing member, and a biasing mechanism must be provided for biasing the positioning member and the releasing member towards each other. Accordingly, a space must be provided to accommodate such axial movement as well as the biasing mechanism. This makes it difficult to produce an axially compact device.